1. Field of the Invention
The present invention relates generally to power supplies and, in particular, to power supplies suitable for operating from a high voltage supply input and for powering low-voltage-rated circuitry.
2. Description of the Related Art
In many applications where high voltages must be regulated or otherwise controlled in some manner, the circuitry for controlling the high voltages must be constructed to withstand high voltages. Since components with a high break-down voltage are expensive when compared to components with lower break-down voltages, there is an advantage is using low break-down voltage components.
FIG. 1 is a diagram of a conventional switching voltage regulator circuit which controls operation of a high voltage, but which uses low break-down voltage components. Further details regarding the construction and operation of the FIG. 1 circuit are disclosed in U.S. Pat. No. 5,313,381 entitled "Three-Terminal Switched Mode Power Supply Integrated Circuit".
As shown in FIG. 1, an AC supply of typically 110 to 120 volts at 50 or 60 Hz is applied to the input of a full wave bridge rectifier BR. One terminal of the output of rectifier BR, which is smoothed somewhat by a capacitor CF, is connected to the primary winding P1 of a transformer T1. Current flow through the primary winding is controlled by an integrated circuit switching circuit 16 which includes a control circuit 14 and a high voltage FET F1. This switching causes an AC component to be introduced so that voltages will be produced at the transformer secondary windings Si and S2. FET F1 can be implemented in discrete form or, together with control circuit 14, as an integrated circuit.
Control circuit 14 includes pulse width modulation (PWM) circuitry which causes FET F1 to periodically switch on and off at a frequency much higher than that of the AC input frequency. Thus, transformer T1 can be made relatively small. Secondary winding S1 is connected to a diode rectifier DA and filter capacitor CA. The DC output produced across capacitor CA is the primary DC power source to be regulated.
An error amplifier 12 compares the DC output with a reference voltage and produces an output based upon the comparison. The error amplifier output drives the input of an opto-coupler 18 which provides electrical isolation between the primary winding P1 circuitry and the circuitry connected to the secondary winding S1. The output of the opto-coupler 18 is thus indicative of the magnitude of the DC output voltage relative to the desired regulated voltage. The opto-coupler output is used to control pulse width modulation circuitry of control circuit 14 so that the duty cycle of the current flow through the primary winding P1 will either increase or decrease thereby altering the magnitude of the DC output voltage.
The switching circuit 16, which forms part of the primary winding P1 circuitry, typically must be powered by a source which is electrically isolated from the secondary winding circuitry S1. A dedicated secondary winding S2 is provided having a rectifier diode DB and filter capacitor CB. The DC output Vbias is connected to the output of the opto-coupler 18. The opto-coupler 18 is conductive a sufficient time to ensure that a capacitor CC remains charged to a voltage Vp. A regulator internal to controller 16 operates to provide an internal regulated voltage from voltage Vp for powering the integrated circuit controller 16. Vp is modulated by error amplifier 12, with the modulation information being used by the pulse width modulation circuitry in control circuit 14 to control the duty cycle of the primary winding P1 current and thus the magnitude of the regulated DC output voltage.
When FET F1 turns off, the inductance of the transformer primary P1 will attempt to maintain a constant current, with the result being that the voltage across the primary abruptly reverses. The drain of FET F1, which is connected to the primary, will increase to a voltage which could easily reach twice the supply voltage. This increase in voltage is due to the combination of leakage inductance and reflected voltage of the secondary winding. Although FET F1 is typically rated to withstand relatively high voltages, as opposed to the components which make up control circuit 14, the FET could be damaged by the very high voltages which could be produced on winding P1.
In order to prevent damage to FET F1 and related circuitry, circuits commonly referred to as a snubber network are typically used in applications such as shown in FIG. 1. FIG. 2 is a diagram of a conventional snubber network which can be used with the FIG. 1 regulator and which limits the voltage across the primary Pi. When FET F1 turns off, the drain voltage will increase in value until the voltage is greater than the supply voltage +V. Diode DC will begin to conduct so that capacitor CD will begin to become charged through resistor RB. Thus, depending upon the values of capacitor CD and resistor RB, the voltage excursion at the drain of F1 is limited to perhaps 50 to 100 volts above the supply voltage +V. Resistor RA operates to discharge capacitor CD after the fly-back pulse has exhausted itself.
Referring again to FIG. 1, the magnitude of the supply voltage VP can be set to a relatively low value by selecting the number of turns in secondary winding S2. Thus, with the exception of transistor F1, the remainder of the circuit controller 16 can be implemented using low voltage circuitry and/or electrical components. However, transformer T1 must include a secondary winding S2 dedicated to producing the supply voltage for the circuit controller 16 while maintaining the necessary electrical isolation from the circuitry powered by the primary DC output voltage by winding S1. Thus, the cost and size of transformer T1 are increased.
The present invention overcomes the above-noted shortcomings of the prior art by providing a low voltage output without the use of a dedicated secondary winding. The low voltage output can be powered by the voltage applied to the transformer primary so that electrical isolation is maintained. Further, the components used in implementing the invention can be made to carry out the snubber network function of FIG. 2. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention.